| Mid-Canada Line | |
|---|---|
 A Piasecki H-21 drops supplies in front of a Mid-Canada Line radar tower. The antennas at the top and bottom of the stack link to the next stations in the line, the three in the middle send data southward to the air defense network. | |
| Active | 1956–1965 |
| Country | Canada |
| Branch | Royal Canadian Air Force United States Air Force |
| Type | Early-warning radar |
| Role | Continental Air Defence |
| Part of | North American Aerospace Defense Command |
The Mid-Canada Line (MCL), also known as the McGill Fence, was a line of radar stations running east–west across the middle of Canada, used to provide early warning of a Soviet bomber attack on North America. It was built to supplement the Pinetree Line, which was located farther south. The majority of Mid-Canada Line stations were used only briefly from the late 1950s to the mid-1960s, as the attack threat changed from bombers to ICBMs. As the MCL was closed down, the early warning role passed almost entirely to the newer and more capable DEW Line farther north.
The MCL was based on the bistatic radar principle, using separated transmitters and receivers. An aircraft flying anywhere between the stations would reflect some of the transmitted signal towards the receiver, where it would mix with the signal travelling directly from the transmitter. The mixing of the two signals produces a pattern that is very easy to detect using simple electronics. As the transmitter is not pulsed, it does not require high voltages and is very simple as well. This leads to a very low-cost system that can cover huge areas, at the cost of providing no information about the precise location of the target, only its presence.
Throughout its history, the MCL suffered from a problem that was never solved; because of the way bistatic radar works, any object relatively close to either station produces a large signal, in contrast to conventional monostatic (single site) radars where this effect is limited to the area immediately around the site. In the case of the MCL, this caused problems when flocks of birds would fly anywhere near either station and swamp the signal of a more distant aircraft. Solving this problem using the Doppler effect was a major design criterion for the AN/FPS-23 "Fluttar" that filled a similar role in the DEW line.